Magnetic particle imaging

ABSTRACT

A Magnetic Particle Imaging (MPI) system with a magnet configured to generate a magnetic field having a field free line, the system including at least one shim magnet configured to modify the magnetic field in a manner to maintain desired magnetic flux distributions during imaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims priority under 35 U.S.C. § 119 to U.S.Provisional Patent Application No. 62/361,463 filed Jul. 12, 2016 andentitled “MAGNETIC PARTICLE IMAGING,” and to U.S. Provisional PatentApplication No. 62/361,475 filed Jul. 12, 2016 and entitled “MAGNETICPARTICLE IMAGING,” the contents of each are hereby incorporated byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant numbers1R43DA041814, 1R43EB020463, and 5R01EB013689 awarded by the NationalInstitutes of Health. The government has certain rights in theinvention.

BACKGROUND

Magnetic particle imaging (MPI) is a technique allowing for thedetection of certain nanoparticles and may be used, for example, indiagnostic imaging applications. Imaging may be facilitated throughmagnets designed to create a Field-Free Region (FFR). Examples of fieldfree regions include a Field Free Point (FFP) and a Field Free Line(FFL).

SUMMARY

A Magnetic Particle Imaging (MPI) system is disclosed. Implementationsmay include a magnet configured to generate a magnetic field having afield free line within the magnetic field, the field free line having anaxis and a center. A first shim magnet may be positioned above the fieldfree line and configured to modify the magnetic field.

In some variations, a second shim magnet can be positioned below thefield free line. The second shim magnet can be configured to modify themagnetic field. The first shim magnet can be a passive shim, an activeshim, or an angled shim magnet pointed generally toward the field-freeline, or elongate.

In other variations, the first shim magnet can configured to decrease agradient along the axis of the field-free line or improve the fidelityof the field-free line.

In yet other variations, the magnet does not include a flux return orcan include a flux return integrated with the magnet where the firstshim magnet may not require water cooling. The first shim magnet can beconfigured to reshape the field free line to approximate an ellipsoidalfield-free region or can be configured for slab imaging.

In other variations, the first shim magnet can be configured to beactively controlled to counteract field free line fidelity degradationresulting from gradient strength changes. The first shim magnet can alsobe configured to be actively controlled to counteract field free linefidelity degradation resulting from shifting of the field free lineduring imaging.

In some variations, a control system can be configured for imaging whileavoiding excitation of a particular portion of a subject. The controlsystem can be configured for increased spatial selectivity in hybridimaging and magnetic actuation MPI.

In yet other variations, a shim magnet can be configured to providelocalized heating and energy deposition in three dimensions when themagnetic particle imaging system is configured for spatially-selectivemagnetic fluid hyperthermia applications.

Implementations of the current subject matter can include, but are notlimited to, methods consistent with the descriptions provided herein aswell as articles that comprise a tangibly embodied machine-readablemedium operable to cause one or more machines (e.g., computers, etc.) toresult in operations implementing one or more of the described features.Similarly, computer systems are also contemplated that may include oneor more processors and one or more memories coupled to the one or moreprocessors. A memory, which can include a computer-readable storagemedium, may include, encode, store, or the like, one or more programsthat cause one or more processors to perform one or more of theoperations described herein. Computer implemented methods consistentwith one or more implementations of the current subject matter can beimplemented by one or more data processors residing in a singlecomputing system or across multiple computing systems. Such multiplecomputing systems can be connected and can exchange data and/or commandsor other instructions or the like via one or more connections, includingbut not limited to a connection over a network (e.g., the internet, awireless wide area network, a local area network, a wide area network, awired network, or the like), via a direct connection between one or moreof the multiple computing systems, etc.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims. While certain features of the currently disclosed subject matterare described for illustrative purposes in relation to particularimplementations, it should be readily understood that such features arenot intended to be limiting. The claims that follow this disclosure areintended to define the scope of the protected subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the description, help explain someof the principles associated with the disclosed implementations. In thedrawings,

FIG. 1 is a diagram illustrating a quadrupolar magnetic field, a FFP,and a FFL in accordance with certain aspects of the present disclosure.

FIG. 2 is a diagram illustrating a simplified MPI system and FFL, asimplified MPI system with a flux return, and two simplified magneticflux paths shown with an exemplary increase in magnetic field gradientdue to the flux return in accordance with certain aspects of the presentdisclosure.

FIG. 3 is a perspective diagram illustrating an exemplary magnetconfiguration for an MPI system including a flux return in accordancewith certain aspects of the present disclosure.

FIG. 4 is a front sectional diagram of the MPI system of FIG. 3illustrating a field free line and two magnetic flux paths in accordancewith certain aspects of the present disclosure.

FIG. 5 is a top sectional diagram of the MPI system of FIG. 3illustrating a flux return that includes a pole piece in accordance withcertain aspects of the present disclosure.

FIG. 6 is a perspective diagram of an exemplary flux return inaccordance with certain aspects of the present disclosure.

FIG. 7 is a simplified diagram of two exemplary laminates that form theflux return of FIG. 6 in accordance with certain aspects of the presentdisclosure.

FIG. 8 is a simplified diagram of an exemplary flux return that includesangled arms in accordance with certain aspects of the presentdisclosure.

FIG. 9 is a perspective diagram of an exemplary MPI system illustratingan example of a shim magnet in accordance with certain aspects of thepresent disclosure.

FIG. 10 is a top sectional diagram of the MPI system of FIG. 9illustrating an example of a shim magnet in accordance with certainaspects of the present disclosure.

FIG. 11 is a side sectional diagram of the MPI system of FIG. 9illustrating an example of a shim magnet in accordance with certainaspects of the present disclosure.

FIG. 12 is a top sectional diagram of an exemplary MPI systemillustrating an example of angled shim magnets in accordance withcertain aspects of the present disclosure.

FIG. 13 is a side sectional diagram of the MPI system of FIG. 12illustrating an example of angled shim magnets in accordance withcertain aspects of the present disclosure.

FIG. 14 is a top sectional diagram of an exemplary MPI systemillustrating an example of an elongate shim magnet in accordance withcertain aspects of the present disclosure.

FIG. 15 is a side sectional diagram of the MPI system of FIG. 14illustrating an example of an elongate shim magnet in accordance withcertain aspects of the present disclosure.

DETAILED DESCRIPTION

An MPI system can be used to image tracer particles that may be presentin an object, for example, in the anatomy of a person or animal. An MPIsystem can image tracer particles by causing them to emitelectromagnetic radiation in response to a locally changing magneticfield. The change in the magnetic field can result from changes in anexternally applied magnetic field, from movement of the tracerparticles, or a combination of the two.

In many implementations, an MPI system will produce magnetic fields thatinclude a field-free region or magnetic null. Tracer particles presentin an object can change the orientation of their magnetic moment as theypass through such a region, or such a region passes through them, andthe magnetic field experienced by the tracer particles changes frombeing oriented in a one direction to being oriented in anotherdirection.

MPI systems typically include a detector configured to detect theelectromagnetic radiation from tracer particles, or detect the changesin magnetic flux resulting the tracer particles responding to changes inthe magnetic field or moving through the magnetic field. Thiselectromagnetic signal can be used to generate an image of the tracerparticles located within an imaging volume.

Some implementations of magnetic particle imaging can include moving theobject to be imaged, moving the location of the field-free region, or acombination of the two.

The distribution of tracer particles imaged in a subject can be relatedto particular anatomical features or physical structures of the object(e.g., particles accumulated in a cavity or blood vessel) or to adistribution of elements in the object that the tracer particles haveattached to (e.g., a particular molecule, cell or tissue type that has apropensity to preferentially bond with the tracer particles or moleculesthat the tracer particles have been attached to or contained within). Inthis way, the determined location of the tracer particles can be used toimage features inside the object.

FIG. 1 is a diagram illustrating a quadrupolar magnetic field, a FFP130, and a FFL 140, in accordance with certain aspects of the presentdisclosure. An MPI system can produce a quadrupolar magnetic field(upper part of FIG. 1) that contains a magnetic null, zero-point orfield-free region 120. In the simplified example of FIG. 1, two coilswith currents traveling in opposite directions are generating aquadrupolar magnetic field. The four “poles” 110 of the quadrupolarmagnetic field are shown by the short arrows. The poles 110 are providedas examples of a magnetic configuration equivalent to the two opposedcoils shown in FIG. 1. The poles 110 are located at points of symmetrybetween the two coils in the case where the currents in the coils areequal and opposite.

In some implementations, the field-free region 120 can be a FFP 130 (asshown by the simplified illustration in the lower left half of FIG. 1).In other implementations, the field-free region can take the form of afield free line 140 (as shown by the simplified illustration in thelower right half of FIG. 1). The Y-axis of the plots in FIG. 1 arelabeled as vertical to be consistent with later figures, showing thetypically vertical orientation of field-free line 140. When an MPIsystem is configured to generate a field-free line 140, the MPI signalis received from the line, instead of from a point. FFL configurationsmay thus utilize projection-based imaging and reconstruction techniques.

Field-free line 140 is a generally elongate region, having a length anda thickness, where the magnetic field is significantly lower than atother locations in the magnetic field generated by the MPI system. Asused herein, a “field-free line” is understood to account for thereality that the line may not be perfectly straight, nor completelyabsent magnetic field, but that such is generally the goal of an FFL.

The field-free line 140 can, in some implementations, be generallyelongate or “linear” only within an imaging volume of the MPI system. Itis less important for the FFL to maintain linearity outside the imagingvolume and thus field-free line 140 may deviate to a different shapeaway from its center, proximate the center of the imaging volume.Similarly, as used herein, a “field-free point” refers to anapproximately spherical region of low magnetic field.

FIG. 2 is a diagram illustrating a simplified MPI system with FFL 140 inthe upper left corner. In the upper right corner, FIG. 2 illustrates anMPI system with a flux return 250 and two simplified magnetic flux paths230 and 240. The bottom of FIG. 2 includes a graph detailing anexemplary increase in magnetic field gradient due to the inclusion offlux return 250 in accordance with certain aspects of the presentdisclosure.

As shown in FIG. 2, the MPI system can include a magnet configured togenerate a magnetic field that has a field-free line 140 within themagnetic field. In the exemplary design shown in FIG. 2, the magnetincludes a first magnet 210 and a second magnet 220. The presentdisclosure, however, contemplates that the requisite magnetic field andfield free line may be generated by any number, and any type, ofmagnets. For example, the magnet may incorporate multiple magnets (2, 3,4, etc.) and such magnets can be, for example, a permanent magnet, acurrent-carrying coil or electromagnet, an electromagnet with a fluxreturn, or any combination of such magnets. The magnet may in fact beonly a single magnet, to the extent such is capable of generating afield free line (for example, a Halbach cylinder). The discussion of theexemplary magnet design herein including two main magnets is notintended to be limiting.

Field-free line 140 is understood to have an axis extending along thelength of field-free line 140 and a center on the axis, which isgenerally understood to be proximate the center of the imaging volume.

The upper right corner of FIG. 2 shows a flux return 250 integrated withthe magnet (here illustrated as including first and second magnets 210and 220). As used in this disclosure, “flux return” refers to anyarrangement of material components that shape the magnetic flux. Fluxreturn 250 may contain, for example, a ferromagnetic material such asiron (or any other material having a low reluctance as compared to othersubstances or as compared to air), to more effectively channel, guide,shape, or concentrate magnetic flux. Flux return 250 may be fabricated,for example, in two separate halves, or in a number of layers oflaminates that can be stacked or otherwise assembled to form flux return250.

Flux returns of the present disclosure can shape magnetic fluxdistributions by way of, for example, creating flux paths of varyingreluctance. For example, a path including a large amount of ironcompared to air will have a lower reluctance than a same-length pathwith less iron and more air. Magnetic resistance, magnetic reluctance,or “reluctance” as used herein, is a concept used in the analysis ofmagnetic circuits. It is analogous to resistance in an electricalcircuit. In likeness to the way an electric field causes an electriccurrent to follow the path of least resistance, a magnetic field causesmagnetic flux to follow the path of least magnetic reluctance. It is ascalar, extensive quantity, akin to electrical resistance. The unit formagnetic reluctance is inverse Henry, H⁻¹.

As shown at the bottom of FIG. 2, the flux guiding and concentratingeffect of a flux return can be leveraged to create larger fieldgradients, in which a stronger field is obtained using a flux return 250as opposed to a magnet system without a flux return. The graph at thebottom of FIG. 2 illustrates the increased gradient in the magneticfield when a flux return is utilized.

Field symmetry and fidelity can be described as how well the fieldrealized by a magnet arrangement matches a desired shape and fluxdensity, e.g., a symmetric FFL 140. As used herein, “magnetic fieldfidelity” or “field fidelity” refers to the quality of the magneticfield pattern as it relates to, for example, the shape and quality ofFFL 140. For example, it may be beneficial to have a highly linear andsymmetric FFL 140, with low gradient along the axis of the FFL 140, butwith high field gradients in the directions orthogonal to the axissurrounding FFL 140.

In one implementation of an MPI system consistent with the presentdisclosure, illustrated in FIG. 3, the assembly includes fourhigh-powered, water-cooled electromagnets where the magnets are elongate(i.e., longer in one dimension than another and not circular). FIG. 3also illustrates an exemplary flux return 250 in accordance with certainaspects of the present disclosure.

In some implementations, two of the magnets may be configured togenerate a homogenous field for shifting the position of FFL 140, forexample, along the Z axis; such magnets may be referred to herein as“shifting magnets” and are labeled Z1 and Z2 (see, e.g., FIG. 5). Mainmagnets, labeled herein as X1 and X2 (see FIG. 5), may also beconfigured to alter the magnetic field in a manner to shift the locationof FFL 140. In other implementations, additional magnets can be used togenerate a fast drive field that acts to rapidly shift the mean positionof FFL 140.

As shown in FIG. 3, the MPI system can also include a bore 310 toreceive a subject to be imaged. The FFL 140 can extend perpendicularlyto the bore 310. In some implementations, the MPI system can beconfigured to rotate about the axis of the bore 310, and in suchimplementations, the orientation of the magnetic fields and FFL 140changes correspondingly.

FIG. 4 is a sectional view of the MPI system of FIG. 3 (denoted by theheavy solid section line in FIG. 3, shown through the middle of thesystem, and corresponding to the X-Y plane in that figure). FIG. 4illustrates a flux return 250, which, in this particular embodiment,surrounds the four magnets. Also shown is FFL 140 and two magnetic fluxpaths 230 and 240 in accordance with certain aspects of the presentdisclosure.

FIG. 5 is another sectional view of the MPI system of FIG. 3 (denoted bythe dashed line around the system in FIG. 3, corresponding to the X-Zplane in that figure). FIG. 5 also illustrates flux return 250 and showsits included pole pieces in accordance with certain aspects of thepresent disclosure. A pole piece 510 (one shown in cross-hatch forillustrative purposes) can pass through the center of, for example, mainmagnets 210, 220, which are shown in FIG. 5 as X1 and X2.

In an exemplary embodiment, flux return 250 may be configured such thata first magnetic flux path 230 at approximately the center of field-freeline 140 has a first reluctance, and a second magnetic flux path 240distal from the center of field-free line 140 has a secondreluctance—and the second reluctance is lower than the first reluctance(see simplified illustrations of flux paths 230 and 240 shown as dashedlines in FIGS. 3 and 4). In this embodiment, it is contemplated that thesecond magnetic flux path 240 “distal from the center of the field-freeline 140,” has a lower reluctance by virtue of the design of flux return250—and not by flux path 240 simply traveling a shorter distance (thatis also at a point distal from the center of FFL 140). In thedescription of this embodiment, it is assumed that magnetic flux paths230 and 240 are each paths that extend out to pass through a pointadjacent field free line 140.

FIG. 6 illustrates one implementation of this exemplary embodiment,through a perspective view of an exemplary flux return 250, while FIG. 7shows two of the three laminates that make up the flux return of FIG. 6.

As shown in FIGS. 6 and 7, flux return 250 can include a pole piece 510having an end, and that end can include a step. To form the step, fluxreturn 250 may include laminations or layers, where a first lamination610 and a second lamination 620 form the step. In this example, firstmagnetic flux path 230 passes through first lamination 610 and secondmagnetic flux path 240 passes through second lamination 620. As shown,the step in pole piece 510 is configured such that the bottom laminate620 protrudes farther toward the imaging volume than the laminate 610 atthe center of pole piece 510, thereby increasing the amount of fluxreturn material (e.g., iron) in the bottom portion of the pole piece andthus decreasing the reluctance at points distal from the center of FFL140. While this reluctance-altering design is shown as creating stepsthrough the use of different shaped laminates, it may also be created bya single pole piece, machined to similar specifications—or by acombination of laminate(s) and machined pole piece(s). While thisimplementation of steps has been shown in relation to pole piece 510, itis contemplated that similar steps on the outer portions of flux return250 (e.g., in the vicinity of labels 610 and 620) may be implemented.

While the reluctance varying properties described above may be createdthrough “steps,” for example in pole piece 510 as described, theseproperties may also be achieved by machining a pole piece to create amore smoothly varying flux return pole profile. For example, pole piece510 may be machined in a continuous or partial curve that likewiseresults in points distal from the center of FFL 140 protruding furthertoward the imaging volume. Smoothly varying pole pieces may take theshape of an arc or chord, a parabolic, hyperbolic, or hyperbolic tangentshape, for example. In yet other implementations, there can be anynumber of laminates or layers of pole pieces of varying length that canbe stacked in order to form a semi-smooth or fine-stepped pole piece.

In yet another implementation, the second reluctance may be lower thanthe first reluctance at least partially by virtue of flux return 250including a lower reluctance material in the vicinity of the secondmagnetic flux path 240 than the reluctance of the material in thevicinity of the first magnetic flux path 230.

In still other implementations, the second reluctance can be lower thanthe first reluctance by virtue of the shape of the flux return incombination with chosen flux return materials.

Flux return 250 can include “flux return arms” (for example, shown as630 in FIG. 6) that can also play a role in shaping or concentratingmagnetic flux, apart from the role that pole piece 510 may play.

FIG. 8 is a simplified diagram of an exemplary flux return 250 thatincludes angled flux return arms 810 in accordance with certain aspectsof the present disclosure. As shown in FIG. 8, a flux return arm can beangled toward the field free line at the imaging volume. This angling isdistinct from flux return arms 710 that are oriented generallyperpendicular to the axis of bore 310, as shown in FIGS. 6 and 7. Suchangled flux return arms can further focus or shape the magnetic flux inthe region of FFL 140 at the imaging volume location. FIG. 8 alsoillustrates magnetic field lines that may result from such angled fluxreturn arms.

Another manner in which the magnetic flux density can be shaped oroptimized for MPI is illustrated in, e.g., FIGS. 6 and 7, wherein polepiece 510 can be fabricated in a manner that includes a taper. Such ataper in pole piece 510 can increase a magnetic flux density near thetaper and proximate the field-free line 140.

The taper referred to herein is (as shown in the figures) taperedtransverse to the FFL 140 or, in other words, the outer edges of pole510 are tapered towards FFL 140. The taper may be linear as shown, orcan be smoothly varying over part or all of its cross-section (or may bea combination of smoothly varying and linear sections). The taper cancome to a point at the end of pole piece 510, or can be only a partialtaper that terminates in a flat end of pole piece 510 (as shown in theexemplary drawings). The taper can be accomplished by any combination ofmachining or the application of different sized layers of materialmaking up pole piece 510 or flux return 250.

Magnetic fields in an MPI system can be modified or further shapedthrough shimming. For example, shimming may be performed to shapemagnetic fields to improve the fidelity of field free line 140 or tochange the shape of field free line 140.

Passive shimming refers to the mechanical attachment of permanentmagnets or other magnetic materials (e.g., iron, steel, mu-metal, etc.)to the MPI system in order to shape the magnetic field. Someimplementations include adding passive shim sets to optimize themagnetic field at particular specific operating points or strengths ofthe main gradient magnet. Such passive shim sets will often sufficientlyoptimize magnetic field distribution over a significant range ofgradient field strengths (e.g., 4-7 T/m). However, when a gradientoperating point outside of the optimized range is desired, a separateset of passive shims may need to be utilized.

Active shimming refers to the powering of electromagnets included withinthe MPI system in order to shape the magnetic field, often in a mannerwhere the power driving a shim magnet is actively changed duringoperation of the magnetic particle imaging system.

The shimming of an MPI system may be performed utilizing active shims,passive shims, or a combination of both.

FIG. 9 is a perspective diagram of an exemplary MPI system illustratingan example of a shim magnet 910 in accordance with certain aspects ofthe present disclosure. FIG. 10 is a top sectional diagram of the MPIsystem, and FIG. 11 is a side sectional diagram of the MPI system.

As shown in FIG. 9, a shim magnet 910 may be centered along the lengthof bore 310, may be aligned essentially in the center of flux return250, and may be positioned between pole pieces 510. As can be seen inFIG. 11, in this location, shim magnet 910 is approximately centeredabove the portion of field free line 140 that is located in the imagingvolume within bore 310 (referred to herein as “above the field freeline”). Shim magnet 1110, also shown in FIG. 11, may similarly be placedat a position that is approximately centered below the portion of fieldfree line 140 that is located in the imaging volume within bore 310(referred to herein as “below the field free line”). In one particularimplementation, shim magnets 910 and 1110 are centered on the axis ofFFL 140 when FFL 140 is located in the center of the imaging volume (inthis case, at the center of bore 310, and centered between magnets 210and 220), thereby allowing for beneficial shaping of FFL 140, asdescribed further below. During imaging, FFL 140 may shift to differentpositions within the imaging volume, and thus shim magnets 910 and 1110would only be approximately centered above FFL 140 at such times.

Although a single pair of opposed shim magnets 910, 1110 are shown inthe exemplary FIGS. 9-11, the present disclosure contemplates that therecan be any number of shim magnets (e.g., 1, 2, 4, 6, etc.), located atany position along bore 310, and at any azimuthal angle around bore 310(e.g., oriented along the X-axis instead of the Y-axis as shown). Inaddition, active shim magnets can be electrically connected to oneanother in series, or parallel, or they can be completely independent ofone another.

FIGS. 12 and 13 illustrate alternative, angled shim magnets 1210, 1220in accordance with certain aspects of the present disclosure. Suchangled shim magnets can be used to further shape or control FFL 140 andcan be located away from (i.e., not on) the axis of FFL 140, butoptionally pointed generally toward FFL 140. Angled shim magnets 1210,1220 can be arranged symmetrically such that the components of themagnetic fields generated in the Z direction can substantially cancel atthe axis of FFL 140. Angled shim magnets 1210, 1220 can have any anglerelative to FFL 140, for example, approximately 45 degrees as shown, 0degrees (i.e. parallel to FFL 140), 30 degrees, 60 degrees, etc. Asshown in FIG. 11, there can also be a similar arrangement of angled shimmagnets 1310, 1320 below bore 310 that are opposite angled shim magnets1210, 1220.

FIG. 14 is a top sectional diagram of the MPI system illustrating anelongate shim magnet 1410 in accordance with certain aspects of thepresent disclosure, and FIG. 15 is a side sectional diagram alsodepicting elongate shim magnet 1410. Elongate shim magnet 1410 can besimilar to shim magnet 910, but elongated along the length of bore 310.As shown in FIG. 15, certain embodiments may also include an opposingelongate shim magnet 1510 below bore 310.

In embodiments of the present disclosure having a shim magnet above theFFL, below the FFL, or above and below the FFL, the shim magnet(s) canbe configured to aid the shaping of the magnetic flux distributionaround the magnet (for example, setting up a desired flux between a polepiece 510 to a flux return arm 630). Shim magnet(s) configured in thismanner can improve the fidelity of the field free line, and can resultin a decrease of the gradient along the axis of the field free line. Inone implementation, the shim magnet(s) can be of sufficient strength tofacilitate a desired magnetic flux distribution such that the mainmagnet does not require a flux return. In another implementation, theflux-facilitating nature of the shim magnet(s) is utilized with a magnetand flux return design facilitating a relatively high fidelity FFL,thereby decreasing the necessary power of the shim magnet(s) to a pointwhere the shim magnet(s) do not require water cooling.

In other implementations, the shim magnet(s) may be used to counteractthe normal flux distribution around the main magnet, in contrast to theconfigurations discussed above that aid in the desired flux distributionfor a high-fidelity field free line. With shim magnet(s) configured inthis counteracting manner, the FFL can be reshaped into different forms,for example, into an approximation of a field-free point, or into anellipsoidal field-free region. In such configurations, an FFL-typemagnet may be configured to behave more like a field free point magnet,thus allowing for improved imaging in situations where signal from partsof a subject are preferably avoided (e.g., when seeking to minimize alarge signal from the liver while trying to detect a small source nearthe liver). A control system associated with the magnetic particleimaging system can thus be configured to avoid excitation of aparticular portion of the subject. In addition, such configurations canallow for increased spatial selectivity in hybrid imaging and magneticactuation MPI systems.

In configurations where shim magnet(s) are configured to counteract thenormal flux distribution around the main magnet, an ellipsoidal fieldfree region can be created, thus allowing for beneficial slab imaging.

In some embodiments, such configurations of shim magnets that counteractthe main magnet flux distribution, may require high power amplifiers,independent drive mechanisms and systems for cooling the shim magnets.

The present disclosure contemplates systems and methods for activelyshimming the magnet (e.g., magnets 520, 530, 540, 550) during an imagingsequence.

Because of the non-linearity of the saturation of a flux return, thefidelity of FFL 140 can vary as magnet gradient strength varies. Forexample, FFL 140 can have a high level of fidelity at a particulargradient strength, but if an imaging sequence requires a change in thegradient strength, resulting changes to the magnetic flux distributioncan negatively impact the fidelity of FFL 140. The present disclosurecontemplates utilization of active shims, as discussed herein, tocounteract the degradation in fidelity with the addition of shim magnetfields.

Similarly, while FFL 140 may have a high level of fidelity when locatedat the center of the imaging volume, as the FFL is shifted away from thecenter during imaging, the FFL can experience a degradation in fidelity.The present disclosure contemplates utilization of the active shims, asdiscussed herein, to counteract this degradation in fidelity.

In certain implementations of the shimming methods and shim magnetsdiscussed herein, main field gradient fidelity can be designed withrespect to a particular design field, and then deviations in strengthand position from this design field can be compensated for by the use ofshim magnet(s). The current in the shim magnets (and other magnets) canbe set using a priori models, measured experimentally and set using alookup table, and/or may be measured and adjusted in real-time using,for example, Hall-effect probes and feedback, or any combinationthereof.

In certain embodiments, shim magnets contemplated by the presentdisclosure may also be configured to provide further localization ofheating and energy deposition in three dimensions, for example, when themagnetic particle imaging system is configured for spatially-selectivemagnetic fluid hyperthermia/excitation applications. For example,reshaping a field-free line to a point-like or ellipsoidal field-freeregion can contain heating or actuation in this smaller region ratherthan along the entire line of a field-free line.

The present disclosure contemplates that the calculations disclosed inthe embodiments herein may be performed in a number of ways, applyingthe same concepts taught herein, and that such calculations areequivalent to the embodiments disclosed.

One or more aspects or features of the subject matter described hereincan be realized in digital electronic circuitry, integrated circuitry,specially designed application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs) computer hardware, firmware,software, and/or combinations thereof. These various aspects or featurescan include implementation in one or more computer programs that areexecutable and/or interpretable on a programmable system including atleast one programmable processor, which can be special or generalpurpose, coupled to receive data and instructions from, and to transmitdata and instructions to, a storage system, at least one input device,and at least one output device. The programmable system or computingsystem may include clients and servers. A client and server aregenerally remote from each other and typically interact through acommunication network. The relationship of client and server arises byvirtue of computer programs running on the respective computers andhaving a client-server relationship to each other.

These computer programs, which can also be referred to programs,software, software applications, applications, components, or code,include machine instructions for a programmable processor, and can beimplemented in a high-level procedural language, an object-orientedprogramming language, a functional programming language, a logicalprogramming language, and/or in assembly/machine language. As usedherein, the term “machine-readable medium” (or “computer readablemedium”) refers to any computer program product, apparatus and/ordevice, such as for example magnetic discs, optical disks, memory, andProgrammable Logic Devices (PLDs), used to provide machine instructionsand/or data to a programmable processor, including a machine-readablemedium that receives machine instructions as a machine-readable signal.The term “machine-readable signal” (or “computer readable signal”)refers to any signal used to provide machine instructions and/or data toa programmable processor. The machine-readable medium can store suchmachine instructions non-transitorily, such as for example as would anon-transient solid-state memory or a magnetic hard drive or anyequivalent storage medium. The machine-readable medium can alternativelyor additionally store such machine instructions in a transient manner,such as for example as would a processor cache or other random accessmemory associated with one or more physical processor cores.

To provide for interaction with a user, one or more aspects or featuresof the subject matter described herein can be implemented on a computerhaving a display device, such as for example a cathode ray tube (CRT) ora liquid crystal display (LCD) or a light emitting diode (LED) monitorfor displaying information to the user and a keyboard and a pointingdevice, such as for example a mouse or a trackball, by which the usermay provide input to the computer. Other kinds of devices can be used toprovide for interaction with a user as well. For example, feedbackprovided to the user can be any form of sensory feedback, such as forexample visual feedback, auditory feedback, or tactile feedback; andinput from the user may be received in any form, including, but notlimited to, acoustic, speech, or tactile input. Other possible inputdevices include, but are not limited to, touch screens or othertouch-sensitive devices such as single or multi-point resistive orcapacitive trackpads, voice recognition hardware and software, opticalscanners, optical pointers, digital image capture devices and associatedinterpretation software, and the like.

In the descriptions above and in the claims, phrases such as “at leastone of” or “one or more of” may occur followed by a conjunctive list ofelements or features. The term “and/or” may also occur in a list of twoor more elements or features. Unless otherwise implicitly or explicitlycontradicted by the context in which it used, such a phrase is intendedto mean any of the listed elements or features individually or any ofthe recited elements or features in combination with any of the otherrecited elements or features. For example, the phrases “at least one ofA and B;” “one or more of A and B;” and “A and/or B” are each intendedto mean “A alone, B alone, or A and B together.” A similarinterpretation is also intended for lists including three or more items.For example, the phrases “at least one of A, B, and C;” “one or more ofA, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, Balone, C alone, A and B together, A and C together, B and C together, orA and B and C together.” Use of the term “based on,” above and in theclaims is intended to mean, “based at least in part on,” such that anunrecited feature or element is also permissible.

The subject matter described herein can be embodied in systems,apparatus, methods, computer programs and/or articles depending on thedesired configuration. Any methods or the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. The implementations set forth in the foregoing description donot represent all implementations consistent with the subject matterdescribed herein. Instead, they are merely some examples consistent withaspects related to the described subject matter. Although a fewvariations have been described in detail above, other modifications oradditions are possible. In particular, further features and/orvariations can be provided in addition to those set forth herein. Theimplementations described above can be directed to various combinationsand subcombinations of the disclosed features and/or combinations andsubcombinations of further features noted above. Furthermore, abovedescribed advantages are not intended to limit the application of anyissued claims to processes and structures accomplishing any or all ofthe advantages.

Additionally, section headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically, and by way of example, although the headings refer to a“Technical Field,” such claims should not be limited by the languagechosen under this heading to describe the so-called technical field.Further, the description of a technology in the “Background” is not tobe construed as an admission that technology is prior art to anyinvention(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the invention(s) set forth in issuedclaims.

The invention claimed is:
 1. A Magnetic Particle Imaging (MPI) systemcomprising: a magnet for the MPI system, the magnet configured togenerate a magnetic field comprising: a field free line within themagnetic field having an axis and a center; and a first shim magnetcentered approximately above the field free line, the first shim magnetconfigured to modify the magnetic field.
 2. The magnetic particleimaging system of claim 1 further comprising a second shim magnetcentered approximately below the field free line, the second shim magnetconfigured to modify the magnetic field.
 3. The magnetic particleimaging system of claim 1 wherein the first shim magnet is a passiveshim.
 4. The magnetic particle imaging system of claim 1 wherein thefirst shim magnet is an active shim.
 5. The magnetic particle imagingsystem of claim 1 further comprising an angled shim magnet pointedgenerally toward the field-free line.
 6. The magnetic particle imagingsystem of claim 1 wherein the first shim magnet is elongate.
 7. Themagnetic particle imaging system of claim 1 wherein the first shimmagnet is configured to decrease a gradient along the axis of thefield-free line.
 8. The magnetic particle imaging system of claim 1wherein the first shim magnet is configured to improve the fidelity ofthe field-free line.
 9. The magnetic particle imaging system of claim 7wherein the magnet does not include a flux return.
 10. The magneticparticle imaging system of claim 7 further comprising a flux returnintegrated with the magnet and wherein the first shim magnet does notrequire water cooling.
 11. The magnetic particle imaging system of claim1 wherein at least the first shim magnet is configured to reshape thefield free line to approximate an ellipsoidal field-free region.
 12. Themagnetic particle imaging system of claim 11 further comprising acontrol system configured for imaging while avoiding excitation of aparticular portion of a subject.
 13. The magnetic particle imagingsystem of claim 11 further comprising a control system configured forincreased spatial selectivity in hybrid imaging and magnetic actuationMPI.
 14. The magnetic particle imaging system of claim 1 wherein atleast the first shim magnet is configured to enable slab imaging. 15.The magnetic particle imaging system of claim 1 wherein at least thefirst shim magnet is configured to be actively controlled to counteractfield free line fidelity degradation resulting from gradient strengthchanges.
 16. The magnetic particle imaging system of claim 1 wherein atleast the first shim magnet is configured to be actively controlled tocounteract field free line fidelity degradation resulting from shiftingof the field free line during imaging.
 17. The magnetic particle imagingsystem of claim 1 further comprising the shim magnet configured toprovide localized heating and energy deposition in three dimensions whenthe magnetic particle imaging system is configured forspatially-selective magnetic fluid hyperthermia applications.